R* & niches (and the meaning of everything)

Ecology Club

11 Mar 10

Markus Eichhorn

Niches Revision

– Classical theory– Modern objections

Empirical niches– Tilman’s R*– ZNGIs– Impact vectors– Supply points

Coexistence criteria

Parallel definitions Species requirements for survival

– Grinnell (1917), Hutchinson (1957) Impacts on the environment

– Elton (1927), MacArthur & Levins (1967)

Hutchinson (1957) Fundamental niche

– Seldom observed

Realised niche– What remains– Implies competition

Dimension 1

Dim

ensi

on 2

n-dimensional hypervolume

MacArthur & Levins (1967)

Empirical frame– Gause’s principle– Lotka-Volterra models– Maximum overlap– Niche packing

Little support– Not falsifiable– Requires evidence of

trade-offs– Predation & stress not

included

What they say…No concept in ecology has been more variously defined or more universally confused than “niche”

Real & Levin (1991)

I believe that community ecology will have to rethink completely the classical niche-assembly paradigm from first principles

Hubbell (2001)

Let’s consider the concept of niche –

If I knew what it meant I’d be rich.

It’s dimensions are n

But a knowledge of Zen

Is required to fathom the b***h

Cottam & Parkhurst in Hurlbert (1981)

Reductionism Plant coexistence

– 3 main resources– High local SR– How to differentiate?

Liebig’s Law (1840)– Most limiting → GR– Animals – usually N

Other forces– Main predators– Environmental stress

Often few factors

Resource availability (R)

Predator density (P)

Per

cap

ita e

ffec

tsBirth rates

Death rates

R* (Tilman 1982)

Resource availability (R)

Per

cap

ita e

ffec

ts

R* R*2

R* definition Minimum R level

– Birth rate = death rate– dN/dt = 0– Population persists

Competition– Lower R* wins– Reduces resources

Other factors– Predation (P*)– Stress (S*)

Predation

Predator abundance (P)

Per

cap

ita e

ffec

ts

P* P*2

Resource A Predator A

Resource (R)

Pre

dato

r B

Pre

dato

r (P

)R

esou

rce

B

Resource (R)

Str

ess

(S)

Niche features

Zero net growth isocline (ZNGI)– Describes organism’s response to environment– Equivalent to Hutchinson’s niche

Impact vectors (I)– Per capita effect of organism on the environment

Supply vectors

Resource A Predator A

Resource (R)

Pre

dato

r B

Pre

dato

r (P

)R

esou

rce

B

Resource (R)

Str

ess

(S)

Resource A Predator A

Resource (R)

Pre

dato

r B

Pre

dato

r (P

)R

esou

rce

B

Resource (R)

Str

ess

(S)

Resource A

Res

ourc

e B

Wins

Wins

Coexist

Resource A

Res

ourc

e B

Wins

Wins

Either wins

Predator A

Pre

dato

r B

Wins

Wins

Coexist

Each species has a stronger impact on the predator to which it is most vulnerable

Resource (R)

Pre

dato

r (

P)

Wins

WinsCoexist

Better defended species (P*↑) must be a poorer resource competitor (R*↓)

Resource (R)

Str

ess

(S)

Wins

Wins

More efficient competitor (R*↑) more affected by stress

Coexisting species1. ZNGIs must intersect

• Otherwise one spp. always wins• Each has an R* advantage

2. Impact vectors must α ZNGIs• Stronger impact on most limiting R• Likely for optimal foraging species• Expend more effort on limiting R

3. Intermediate supply vector• Depends on position of supply point• Intraspecific competition > interspecific

Implications No. spp. = no. limiting resources / predators

– Local coexistence only– –ve feedback between requirements & impacts

Regional coexistence through habitat heterogeneity

Predictions1. Spp. with lowest R* best competitor for that R2. Dominance varies with ratio of 2 R3. No. spp. ≤ no. limiting R4. R supply vector → outcome5. Impact vectors → outcome6. Coexistence along a gradient through trade-offs7. Highest SR at intermediate ratio of 2 R

Few tests in animal systems

Most in plants / microbes

R* evaluation

Plant v. animal ecologists– Difference largely due to tradition & inertia– Predictions supported but more evidence needed

41 R* tests → 39:1:1 (Wilson et al. 2007)

Supported? Producer 1° consumer Detritivore

Yes 22.5 5.5 3

No 8.5 1.5 1

Miller et al. (2007)

Tilman (1977)

0 20 6040 80 100

1

2

3

4

5Cyclotella and Asterionella

2 essential Rs

SiO2 (μM)

PO

4 (

μM

)

Tilman (1982)

Park Grassland Experiment

Grasshopper diets

Behmer & Joern (2008)Same diet, different optima

Serengeti browsers

Stem

Leaf

Topi v. Wildebeest – unstable equilibrium

Serengeti ungulates

Cell wall biomass

Cel

l con

tent

s bi

omas

s

Large species win when lots of cell wall

Small species when high quality forage

Murray & Baird (2008)

Resource A

Res

ourc

e B

Resource A

Res

ourc

e B

Resource A

Res

ourc

e B

Excluded species Invasive species

Predator A

Pre

dato

r B

Coexistence through variable predator densities

Resource

Pre

dato

r

Resource

Pre

dato

rGradient replacement due to either P or R

Resource

Str

ess

No effect of varying R

e.g. rocky shore seaweed species & desiccation

Nitrogen

Ligh

t

Pioneers

Competitors The successional niche

Nitrogen

Ligh

t

Facilitation

Nitrogen

Ligh

t

Increased light competition

New niche theory1. Joint description of the environmental conditions

that allow a local population to persist and the per capita effects on the environment

2. The ZNGI of an organism, combined with the impact vectors on the ZNGI in the multivariate space defined by the environmental factors

Chase & Leibold (2003)